Compression molding is a technique for forming parts wherein a charge is placed between upper and lower heated die members defining a mold cavity. The dies are then brought to a closed position where the dies compress the charge causing it to flow and fill the mold cavity. After the thermoset resin cures, the molds are opened and the finished part removed.
Compression molding techniques have been used to make parts having a relatively flat surface, such as exterior automotive body panels. The charges used for making such articles include a thermosetting resin containing reinforcing fibers such as glass fibers, polyaramide fibers or carbon fibers and various fillers such as calcium carbonate and carbon black. A charge is often formed into a sheet molding compound (SMC). Compression molding has been limited in the articles that can be made owing to the inability to control porosity and pitting of the article surface that lead to paint defects and high scrap rates. It is commonly believed that such defective areas result primarily from degassing of trapped air that is trapped in the charge during molding.
The use of vacuum during the compression molding process improves the surface qualities of the article as detailed for example in U.S. Pat. Nos. 3,840,239; 4,488,862; and 6,805,546; yet surface finish problems continue to limit the articles that are efficiently formed through compression molding. In an effort to fix surface quality problems in-mold coating techniques have been explored as detailed for example, in U.S. Pat. No. 4,081,578. This method employs an additional processing step where the cured article remains in the mold and is coated with a composition that spreads and penetrates the surface to fill surface defects. This corrective technique met with limited acceptance owing to lower throughput and environmental weathering concerns.
Compression molding and corrective in-mold coating become even more difficult as articles become more three dimensional in contours, along with greater differentials in article thickness throughout the part associated with ribs, bosses and other projections.
A conventional process of compression molding a sheet molding compound uses heated dies which when closed form a mold. The compound is placed in the mold cavity when the dies are open and the dies are moved closer to one another to form a partially closed position to engage a vacuum seal sheath that includes the mold cavity and surrounding area to form a vacuum chamber, without the upper die contacting compound contained within the mold cavity. The vacuum chamber is then typically evacuated to a reduced pressure of less than 7 inches of mercury absolute (24 kPa) followed by closing the dies to a fully closed condition. Upon fully closing the dies to define the mold cavity, the sheet molding compound spread to fill the mold. The mold cavity is then returned to atmospheric pressure while the molding pressure is maintained on the part during the remainder of the curing cycle. After the part is thoroughly cured, the dies are opened allowing the removal of the finished part. This approach has proven only partially effective in controlling article surface porosity and pitting.
Thus, there exists a need for a process of vacuum compression molding that controls the rate of movement of the molding dies relative to another and vacuum pressure to provide for kinetically controlled degassing during compression molding to afford improved molded article surface quality.